The present invention relates generally to encapsulants for optoelectronic devices and, more particularly, to such encapsulants exhibiting superior optical, thermo-mechanical, electrical, and environmental qualities. The present invention also relates to methods for making such encapsulants.
Optical and electronic (i.e., “optoelectronic”) devices, such as LEDs, photodetectors, and fiber optic components, generally are encapsulated using a variety of materials to protect the devices from vibration, humidity, heat, environmental deterioration, electrical leakages, and other deteriorative factors.
U.S. Pat. No. 3,547,871 to Hofman et al. describes a low coefficient of thermal expansion (“CTE”) encapsulant comprising an epoxy resin and a filler having a particle size ranging between 10 μm and 300 μm. This encapsulant resin has a viscosity below 20,000 cP, at 100° C. The filler is selected from silica, fused quartz, beryllium aluminum silicate, lithium aluminum silicate, and mixtures of these. The encapsulants have a CTE lower than 50 ppm/° C. The Hofman '871 patent does not disclose the encapsulant's light transmittance.
U.S. Pat. No. 5,175,199 to Asano et al. describes a sol-gel method for making a multi-component glass filler to be mixed into a transparent epoxy, which can be used as an encapsulant for optical semiconductor devices. In this method, a TiO2—SiO2 gel is synthesized by hydrolyzing and condensing a silicon alkoxide and a titanium alkoxide. This gel is dried, and then either ground into particulate matter, followed by sintering to dense glass beads, or sintered into a dense glass followed by grinding into glass beads. This sintering is achieved at a very high temperature range of 1,050° C. to 1,250° C. The disclosed filler manufacturing method has the disadvantages of being complicated and expensive, as well as requiring lengthy preparation time and high sintering temperature. Additional disadvantages include possible phase separation, crystallization, and coloring of TiO2 at the high sintering temperatures required using the method. Phase separation and crystallization cause intolerably high refractive index differences between the glass filler and the epoxy, thereby lowering the transmittance. TiO2 also is known to possibly cause yellowing of organic resins in which it is included as a result of extended exposure to light. This leads to degradation of the transmittance of the encapsulant over time.
U.S. Pat. No. 5,198,479 to Shiobara et al. uses the method described in the Asano et al. '199 patent described above, and it further discloses a method to overcome the discoloration problems of the TiO2—SiO2 fillers discussed above, by the addition of organic phosphorus anti-discoloring agents into the uncured epoxy-filler composition. This addition, while effective, further complicates use of the method, and the resulting filler therefore is even more expensive to manufacture.
European Patent No. 0 391 447 B1 to Nakahara et al. teaches a sol-gel method for the production of multi-component metal-oxide particles that can be used as fillers in transparent organic resins. The Nakahara et al. method incorporates steps of first preparing seed particles of single-component metal oxides and then growing these particles by adding hydrolyzable and condensable organic metal compounds such as metal alkoxides, to prepare multi-component particles such as TiO2—SiO2, ZrO2—SiO2, and Al2O3—SiO2. This process is considered unduly complicated and expensive.
U.S. Pat. No. 5,618,872 to Pohl et al. describes a method for making multi-component encapsulant filler particles for optoelectronic devices, comprising two or more oxides selected from SiO2, TiO2, ZrO2, Al2O3, V2O5, and Nb2O5. The method for particle preparation described in this patent is similar to that described in the Nakahara et al. patent, and it shares the same drawbacks.
Dunlap and Howe, in Polymer Composites, vol. 12(1), pp. 39-47, (1991), describe a casting composition comprising a resin and an index-matched filler prepared by ball-milling of a glass. The size of the filler particles ranges between 2 μm and 100 μm. Subsequent to ball-milling, the filler particles are annealed at temperatures between 0° C. and 10° C. above the glass strain point for at least one hour, to remove stresses as well as organic contaminants. The inventors have found that heat treatments at temperatures above the strain point of the glass can reduce transmittance, and therefore such temperatures should be avoided.
Japanese Patent Publication No. 11-074424 to Yutaka et al. describes a method for making an encapsulant for use in a photosemiconductor device. In this method, a silica powder containing PbO or TiO2 having a particle size ranging between 3 μm and 60 μm is used as an index-matched filler for an epoxy resin composition. PbO or TiO2 can cause crystallization during manufacturing of these multi-component glass fillers, thereby decreasing the transmittance of the filled epoxy. In addition to the aforementioned disadvantages of using TiO2 as a filler material, PbO is known to be a health hazard, and its use in manufacturing of the encapsulants should therefore be avoided.
U.S. Pat. No. 6,246,123 to Landers et al. describes an encapsulant having high transmittance, low CTE, and low Tg. The encapsulant is made from a polymer resin and an index-matched filler. However, the filler is selected from a group consisting of alkali zinc borosilicate glasses. The presence of alkali ions is known to potentially reduce electrical resistivity of encapsulants, leading to high leakage currents and possible damage to the encapsulated device. For example, U.S. Pat. No. 4,358,552 to Shinohara et al. explains that encapsulants incorporating low levels of alkali contaminants, such as Li+, Na+, K+, and ionic contaminants such as Cl, improves electrical insulation of the encapsulated electronic device.
Naganuma et al. in Journal of Material Science Letters, vol. 18, pp. 1587-1589, (1999), describes preparation of encapsulants for optoelectronic devices by mixing an epoxy with a filler prepared from a multicomponent glass, SiO2—Al2O3—B2O3—MgO—CaO having an average particle size of 26 μm or 85 μm. During the described curing of the epoxy and the filler mixture, the mold is turned over every 10 minutes to prevent segregation of the filler.
Japanese Patent Publication No. 2001-261933 to Yamada et al. describes an epoxy resin composition for making an encapsulant for use in a photosemiconductor device, comprising an epoxy resin, a hardener, an accelerator, and a glass filler. The refractive index of the glass filler is adjusted to match that of the epoxy resin by using multi-component glasses that are prepared by blending oxides such as Na2O, Al2O3, CaO, BaO, ZnO, TiO2, and B2O3.
Japanese Patent Publication No. 2002-105291 to Komori et al. likewise describes an epoxy resin composition for making an encapsulant for use in a photosemiconductor device, the epoxy resin composition comprising an epoxy resin, a hardener, an accelerator, and a glass filler. The refractive index of the glass powder is adjusted to match that of the epoxy resin by using multi-component glasses that are prepared by blending oxides such as SiO2, Al2O3, and CaO. Komori et al. specifies that the filler should be in spherical shape and particle size of this filler should be in the range of 5 μm to 100 μm.
It should be appreciated from the foregoing description that there remains a need for an improved transparent encapsulant suitable for manufacturing of optoelectronic devices, which avoids the drawbacks of the encapsulants and manufacturing methods described above. The present invention fulfills this need and provides further advantages.